Photometric measurements are used inter alia for determining concentration. If light is passed through the solution of an absorbing substance, then the intensity of the transmitted proportion is dependent, according to the Beer-Lambert law, on the material properties of the substance, the concentration and the path length through which the light passes.E=ε*c*d                 E=extinction (logarithm of the quotient of the intensity of the incident and transmitted light proportion)        ε=extinction coefficient (substance-specific)        c=concentration of the substance        d=path length of the solution through which the light passes        
Concentration determinations are usually carried out by mathematically ascertaining the ratio of known substance concentrations c to their extinction E and thus being able to calculate unknown concentrations by reference to extinction measurements. This means in practice often: for the photometric measurement, a specified volume of the reagent causing a coloration with the substance to be determined is combined with a specified volume of a solution of the substance of unknown concentration and mixed and the extinction E (absorption) of the resulting coloration is measured photometrically in a vessel with specified dimensions (measuring cell with path length d), where the unknown concentration of the sample is ascertained by reference to the extinction value by comparison with calibration values of known concentrations.
The substance-specific extinction coefficient ε is often configured for customary photometric measurement methods in such a way that only samples with a relatively low concentration c can be measured. This means the realizable measurement ranges of this method are limited on the part of the applicability of the Beer-Lambert law, the substance-specific properties, and the apparatus possibilities of the measurement systems used. An extension of the measurement range is usually realized through a dilution of the substance preceding the actual measurement or a reduction in the path length d.
EP 1 825 915 A2 relates to a handheld device for metering and optionally titrating. A piston, moved by step motors, drives sample fluid into a vessel where the other part of the titration partner is located. A color change is ascertained with a sensor and controls the step motor of the handheld device in order to find the change point with higher accuracy. The document does not report anything about using samples with high concentrations.